Study On The Structure And Properties Of Laser Cladding NiCoCrAlYSi Coating Under The Action Of High Current Pulsed Electron Beam

In order to meet the requirements of high efficiency and high thrust-to-weight ratio of engines in the aviation and marine fields,the inlet temperature of the turbine has reached or far exceeded the maximum operating temperature of the high-temperature blades.In this extreme operating environment,the high-temperature blades cannot guarantee their mechanical and chemical stability.For this reason,the simplest and most effective method is to prepare a high-temperature protective coating on the surface of the material.MCrAlYX coating has gradually become one of the most widely used systems for high temperature protective coatings due to its excellent comprehensive properties such as high temperature resistance and corrosion resistance.After long-term service,inevitable spalling failure will occur of these tailor-made coatings for superalloys.The stable growth of the thermally grown oxide（TGO）on the surface of the MCrAlYX coating plays a vital role in its high-temperature service life.The stable growth of TGO is closely related to the microstructure,phase structure and grain size of MCrAlYX coating,and the preparation method and surface modified technology of the coating determine its microstructure and performance characteristics.Therefore,choosing appropriate MCrAlYX coating preparation method and surface modified technology is of great significance to improve the high temperature service performance of the coatings.In this paper,laser cladding（LC）technology was used to prepare the NiCoCrAlYSi coating,and then the surface of the samples was irradiated by high-current pulsed electron beam（HCPEB）device.X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersive spectrometer（EDS）,transmission electron microscopy（TEM）and other devices were used to characterize the microstructural evolutions and the growth processes of TGO of the cladding coating.The process reveals the modification mechanism of NiCoCrAlYSi cladding coating under HCPEB irradiation and the enhancing mechanism of the high-temperature oxidation resistance of NiCoCrAlYSi cladding coating based on microstructural control.Through complete randomalized design of laser cladding,the metallurgical defects and solidification microstructure characteristics of NiCoCrAlYSi cladding coatings were studied under different laser cladding parameters,in order to produce superior cladding coatings via parameter optimization.The microstructural analysis shows that the NiCoCrAlYSi cladding coating was mainly composed ofγ/γ′phase.Many pores and typical dendritic segregation were found on the coating surface.After HCPEB irradiation,the cladding metallurgical defects disappeared.The surface was remelted,and the thickness of the remelted layer was gradually increased with the increase of HCPEB pulses.Besides,cross-slip and nanocrystalline structures were induced on the irradiated surface.The surface micro-hardness results show that the surface hardness of the cladding coating after HCPEB treatment was significantly higher than that of the original one.The results of high temperature oxidation resistance show that after 10 hours oxidation,TGO with loose pores and microcracks was formed on the surface of NiCoCrAlYSi cladding coating.The composition and thickness of TGO were very uneven,and a small amount of dot-like internal oxidation occurred inside the cladding coating.After 40 hours oxidation,spinel clusters appeared on the cladding coating.Phenomena of cracking and even spalling were obvious of these spinels,leaving pits with a three-layered structure existed.After 70 hours oxidation,the thickness of TGO was increased significantly,mainly due to the fast growth of the mixed oxide.The cladding coating had a severe internal oxidation phenomenon,and the internal oxidation depth reached to 15μm.After 100 hours oxidation,the thickness of TGO begun to decrease,and large-scale cracking and deep-level spalling appeared on the surface.The internal oxidation was further intensified and gradually connected into pieces.The substrate of the partially exfoliated area was directly exposed,and the protective effect of TGO completely failed.As for 15-pulsed HCPEB irradiated cladding coating,after 10 hours oxidation,a double-layered TGO was formed,which was composed of Al₂O₃ and mixed oxides.The TGO/cladding interface was flat and dense.As the oxidation progressed,the mixed oxides of the TGO gradually shattered from flat to granular,and a small number of pores were generated.Due to the formation of a flat and dense Al₂O₃ protective film on the surface of the 15-pulsed irradiated cladding coating,the thickness of the TGO increased from 1.8μm（after 10 hours oxidation）to 4.1μm（after 100 hours oxidation）.TGO had a good protection performance,and the high-temperature oxidation resistance of the cladding coating was significantly improved.As for 45-pulsed HCPEB irradiated cladding coating,after 10 hours oxidation,a double-layered TGO was also formed.A compact Al₂O₃ layer was mainly existed in the TGO/cladding coating interface,while a complex mixed oxide layer covered the Al₂O₃layer.After 40 h,the subsurface Al₂O₃ was completely covered by the mixed oxide.After 70 h of oxidation,the TGO of the double-layer structure became uneven,with spinels appearing on the surface,and the cracks were initiating,with spalling waiting to happen.After 100 hours of oxidation,TGO was still a double-layered structure,cracking and spalling occurred in some areas,and the spalling area showed a four-level step distribution.The results show that the 45-pulsed irradiated cladding coating had a certain improvement to high-temperature oxidation,but weaker than that of 15-pulsed irradiated one.The multiple pulses of HCPEB irradiation on the surface of the cladding coating can promote the rapid formation of protective oxide film during the oxidation process,improve its chemical stability,and effectively enhance the high temperature oxidation resistance of NiCoCrAlYSi cladding coating.